One of the disadvantages of plastic materials is their transparency to electromagnetic radiation. The plastic enclosures permit the electromagnetic (EM) radiation to escape from their source and cause EM pollution and interference with other electric or electronic equipment. The problem of electromagnetic interference is referred to as EMI. Also the plastic enclosures or housings do not protect sensitive and critical electric/electronic equipment such as computers, navigation instruments, medical equipment, radio and television receivers, cardiac pacemakers, audio and high fidelity equipment, etc. from EMI coming from the outside.
Heretofore, to prevent EMI, the surface of the plastic material has been coated with a conductive material, to provide shielding properties, such as by the application of electrically conductive silver, nickel, copper and graphite coatings, or the use of a metal or screen. Such secondary processes shield the sensitive equipment from outside EMI and also contains the EMI generated inside the enclosure and thus prevent the pollution to the surroundings. While these shielding processes are useful, they are time-consuming; they require extra labor and are costly; and they do not lend to automation in the processing cycle. They also present adhesion problems of the coatings.
It has now been found that the above-mentioned secondary processes, and the disadvantages which attend them, can be eliminated by incorporating into polyphenylene ether-styrene resin blends such conductive materials as aluminum flakes, combinations of aluminum flakes with carbon fibers or conductive carbon black, or a combination of carbon fibers with conductive carbon black or carbon fibers. The resulting composites when molded have been found to be highly useful for EMI shielding while at the same time maintaining the physical properties of the polyphenylene ether-styrene resin blends or compositions.
The polyphenylene ether resins are a family of engineering thermoplastics that are well known to the polymer art. These polymers may be made by a variety of catalytic and non-catalytic processes from the corresponding phenols or reactive derivatives thereof. By way of illustration, certain of the polyphenylene ethers are disclosed in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and in Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358. In the Hay patents, the polyphenylene ethers are prepared by an oxidative coupling reaction comprising passing an oxygen-containing gas through a reaction solution of a phenol and a metal-amine complex catalyst. Other disclosures relating to processes for preparing polyphenylene ether resins, including graft copolymers of polyphenylene ethers with styrene type compounds, are found in Fox, U.S. Pat. No. 3,356,761; Sumitomo, U.K. Pat. No. 1,291,609; Bussink et al, U.S. Pat. No. 3,337,499; Blanchard et al, U.S. Pat. No. 3,219,626; Laakso et al, U.S. Pat. No. 3,342,892; Borman, U.S. Pat. No. 3,344,116; Hori et al, U.S. Pat. No. 3,384,619; Faurote et al, U.S. Pat. No. 3,440,217; and Cooper et al, U.S. Pat. No. 3,661,848 U.S. Pat. No. 3,733,299, U.S. Pat. No. 3,838,102 and U.S. Pat. No. 3,988,297. Disclosures relating to metal based catalysts which do not include amines, are known from patents such as Wieden et al, U.S. Pat. No. 3,442,885 (copper-amidines); Nakashio et al U.S. Pat. No. 3,573,257 (metal-alcoholate or -phenolate); Kobayashi et al, U.S. Pat. No. 3,445,880 (cobalt chelates); and the like. In the Stamatoff patents, the polyphenylene ethers are produced by reacting the corresponding phenolate ion with an initiator, such as peroxy acid salt, an acid peroxide, a hypohalite, and the like, in the presence of a complexing agent. Disclosures relating to non-catalytic processes, such as oxidation with lead dioxide, silver oxide, etc., are described in Price et al, U.S. Pat. No. 3,382,212. Cizek, U.S. Pat. No. 3,383,435 discloses polyphenylene ether-styrene resin compositions. All of the above mentioned disclosures are incorporated herein by reference.
It is the primary objective of this invention to provide novel thermoplastic molding compositions of polyphenylene ether-styrene resin compositions which have incorporated an amount of a conductive material, namely, aluminum flakes, or aluminum flakes in combination with carbon fiber or with conductive carbon black, or with a combination of carbon fibers and carbon black, or carbon fibers so as to render the resin EMI shielding.